DateTime is probably not the best module to use, particularly as you are going forward and don't need dates before the start of the Gregorian calendar. The man page for DateTime notes that it does not account for leap seconds, so your results may be out.
The default time zone for new DateTime objects, except where stated otherwise, is the "floating" time zone. This concept comes from the iCal standard. A floating datetime is one which is not anchored to any particular time zone. In addition, floating datetimes do not include leap seconds, since we cannot apply them without knowing the datetime's time zone.
The results of date math and comparison between a floating datetime and one with a real time zone are not really valid, because one includes leap seconds and the other does not. Similarly, the results of datetime math between two floating datetimes and two datetimes with time zones are not really comparable.
As suggested by thomas895 create your new epoch seconds for the new dates given, calculate the epoch seconds for 01 January 2000, noon, and subtract it from the epoch seconds for new dates given.
The number of seconds difference between future dates will be the same whatever the base epoch date is.